ABSTRACT Osteoporosis is a major health problem for the US aging population and globally. Osteoporosis results from decreased bone formation by osteoblasts and/or increased bone resorption by osteoclasts. Treatment of osteoporosis is directed at stimulating new bone formation and/or inhibiting resorption of old bone. While several bone resorption inhibitors are available for clinical use, very few therapeutics that stimulate bone formation are known. Parathyroid hormone (PTH) and PTH-related peptide (PTHrP) activates a common receptor in osteoblasts, PTH1R, and stimulate both bone resorption and formation. When administered in the correct regimen, PTH results in a net increase in bone mass, however, its effects plateau in 2 years;this limits its clinical use. Understanding the molecular and cellular targets involved in the anabolic actions of PTH is critical for optimizing its therapeutic use and for the development of novel agents that promote bone formation. Our data suggest that, in osteoblasts, PTH and PTHrP regulates the expression and/or the activity of cyclin D1, the cell division kinase, CDK1, MAP kinase (MAPK), MAPK phosphatase-1 (MKP-1) and JunB. These data led to the hypothesis that PTH/PTHrP stimulated increase in MKP-1 expression initiates a cascade of events that eventually suppress the proliferation and promotes the differentiation of mature osteoblasts. We propose to examine the hypothesis that MKP-1 is involved in the anabolic actions of PTH and PTHrP. We plan to use multiple in vitro and in vivo approaches of osteoblast cell lines and primary osteoblast cultures, MKP-1 knockout (KO) mice, an engineered bone growth model of ossicles populated with osteoblasts derived from bone marrow stromal cells (BMSCs) from wild type or MKP-1 KO mice and implanted in nude mice, and transgenic mice expressing a constitutively-active PTH1R in osteoblasts in the MKP-1 KO background. Specific Aim 1 examines the role of MKP-1 in PTH and PTHrP action in vitro at different stages of osteoblast maturation. Specific Aim 2 examines the role of MKP-1 in PTH/PTHrP action in vivo using the MKP-1 knock out mice, the ossicle model, and constitutively active PTH1R transgenic mice in the MKP-1 KO background. Specific Aim 3 examines the hypothesis that MKP-1 deficiency limits PTH/PTHrP anabolic actions in bone in ovariectomized mice;the MKP-1 knock out mice will be challenged with traditional ovariectomy to induce osteopenia and PTH/PTHrP effects will be evaluated. These experiments shall promote our understanding of the anabolic actions of PTH and PTHrP and may identify potential targets for novel therapeutic agents for the treatment of diseases which adversely affect bone such as osteoporosis, secondary hyperparathyroidism, periodontal diseases or craniofacial abnormalities.